Evaluation of Long-term Discharge in Swedish Rivers

Sammanfattning: Seasonal forecasting is not an easy task to deal with. A number of large scale atmospheric phenomena, e.g. Northern Atlantic Oscillation (NAO), Scandinavia (SCA) and Pacific Decadal Oscillation (PDO) affect the climate in Sweden and the effect on rivers is an integration of all the complex inter-relation among all these atmospheric phenomena. In this Master thesis, the river discharge from 107 stations, distributed over Sweden, were examined in the search for decadal oscillation. River discharge measurement data, collected from the Swedish Meteorological and Hydrological Institute, span periods between 40 to 100 years. In order to examine these time series, the Wavelet analysis was used. The discharge stations were divided into groups, depending on location and wavelet spectrum and these groups were compared to the wavelets of the climate indices that represent the atmospheric phenomena. Results show the presence of decadal oscillations in the river discharge in Sweden. It was also possible to make a rough grouping of stations with common features in their river discharge wavelet spectrum. Such groups followed roughly the climate zone distribution. Possible connections to climate indices were found, of which Pacific Decadal Oscillation (PDO) and Pacific/North American (PNA) were the two most evident. One thing to note is that SCA was the climate index that was least connected to the discharge. Why river discharge tends to depend more on PDO and PNA than on SCA, when SCA is the Scandinavian affecting pattern, could be due to the vast amount of decadal and multidecadal oscillations that PDO and PNA have, which also many of the river discharge time series have.